Air filtration media generally relies on particle capture by contact with a filter media. This can occur by random impaction by a number of impaction mechanisms (e.g., direct interception, inertial impaction or diffusion) or by actively attracting particles to the media, generally by electrostatic attraction.
Filtration media relying on random impaction events generally show increased efficiency by increasing the surface area of the filtration media while decreasing the size of any open spaces available for fluid to pass without impacting the media. Generally, the most cost effective and common approach has been the use of fibrous filter media, particularly nonwoven fibrous filter media. Fluid is passed through the filter media resulting in particle capture by impaction with the fibrous media. The drawback of this approach is that higher capture efficiencies are typically realized at the expense of a high pressure drop created by the filter media flow resistance, which flow resistance can increase significantly over the lifetime of the filter.
Filtration relying exclusively on electrostatic attraction is exemplified by active charged conductive electrode plates separated by dielectric insulators such as described in U.S. Pat. Nos. 4,234,324 or 4,313,741. With these devices, inherently charged particles or particles induced with a charge, such as by an ionizer, are passed between flat charged electrode plates. These devices generally have very low pressure drops but are ineffective against uncharged particles and require a power source.
In order to capture some of the benefits of these electrostatic devices, it has also been proposed to induce permanent or temporary charges on a fibrous filter media by a variety of methods. Charged fiber filter media have increased efficiencies relative to comparable uncharged fiber filter media but when used in conventional flow through type filters, still have significant pressure drops due to flow resistance.
As a method of decreasing this flow resistance and associated pressure drop, it has been proposed to create flow through channel filters where the flow channels sidewalls are formed by particulate or sorbent filter media. Particles are captured when they contact these filter media sidewalls. In view of its increased particle capture capabilities, generally the particulate filtration media is an electret charged fibrous media, generally a nonwoven filter media formed of charged fibers. For example, Japanese Kokai 7-144108 (published Jun. 6, 1995) indicates that it is known to form honeycomb filters (e.g., pleated corrugated filter media resembling corrugated cardboard) from electret charged nonwoven filter media. This patent application proposes increasing the long term efficiency of such a filter structure by forming it from a filter media laminate of charged meltblown fiber filter media and charged split fiber filter media (e.g., similar to that disclosed in U.S. Pat. No. RE 30,782). Japanese Kokai 7-241491 (published Sep. 19, 1995) proposes a honeycomb filter, as above, where the pleated layers and the flat layers forming the corrugated honeycomb structure are alternating layers of electret charged nonwoven filter media and sorbent filter media (activated carbon loaded sheet or the like), the activated carbon layer preferably is formed with a liner (e.g., a nonwoven) that may also be electret charged. Japanese Kokai 10-174823 (published Jun. 30, 1998) discloses another honeycomb type filter, as above, where the filter material forming the honeycomb structure is formed from a laminate of an electret charged nonwoven filter layer and an antibacterial filter layer. These honeycomb type filters are advantageous for uses where low pressure drop is critical and single pass filtration efficiency is less important; for example, recirculating type filters such as used in air conditioners, room air cleaners or the like. Generally, these honeycomb filters are formed by a process similar to that used to form cardboard where one filter media is pleated and glued at its peaks to a flat layer. The assemblies are then stacked or rolled up where adjacent laminate layers can be joined by glue or hot melt adhesive. The filtration media is charged by conventional techniques prior to forming the honeycomb structures.
A different approach to a flow through type filter is proposed in U.S. Pat. No. 3,550,257 where the charged filtration media is a film rather than a nonwoven filter media. The charged films are separated by spacers strips that are described as open cell foam webs of glass fibers or corrugated Kraft paper. The pressure drop is described as dependent on the porosity of the spacers and the space between the charged dielectric films. Japanese Kokai 56-10314 (published Feb. 2, 1981) discloses a similar structure where a corrugated honeycomb structure is formed with either or both the pleated or flat layers formed from a charged polymeric film (defined as a film or a nonwoven). The layers are adhered by melting the front edges of the multilayer structure together. It is disclosed that the film is imparted with "wrinkles" by the folding process. Similar film type honeycomb structures formed from charged films are further disclosed in related Japanese Kokai 56-10312 and 56-10313, both published Feb. 2, 1981.
Although these honeycomb structures are advantageous in view of their low pressure drop, they are often difficult to manufacture, particularly with lower basis weight nonwoven webs and can be structurally unstable. Further, there is a need for general improvements in filtration efficiency with these structures.